1. Technical Field
This invention relates to an improved composition and process for the chemical mechanical polishing or planarization of semiconductor wafers. More particularly, it relates to such a composition and process which are tailored to meet more stringent requirements of advanced integrated circuit fabrication.
2. Background
Chemical mechanical polishing (or planarization) (CMP) is a rapidly growing segment of the semiconductor industry. CMP provides global planarization on the wafer surface (millimeters in area instead of the usual nanometer dimensions). This planarity improves the coverage of the wafer with dielectric (insulators) and metal substrates and increases lithography, etching and deposition process latitudes. Numerous equipment companies and consumables producers (slurries, polishing pads, etc.) are entering the market.
CMP has been evolving for the last ten years and has been adapted for the planarization of inter-layer dielectrics (ILD) and for multilayered metal (MLM) structures. During the 80""s, IBM developed the fundamentals for the CMP process. Previously (and still used in many fabs today) plasma etching or reactive ion etching (RIE), SOG (xe2x80x9cspin on glassxe2x80x9d), or reflow, e.g., with boron phosphorous spin on glass (BPSG), were the only methods for achieving some type of local planarization. Global planarization deals with the entire chip while xe2x80x9clocalxe2x80x9d planarization normally only covers a xcx9c50 micron area.
At the 1991 VMIC Conference in Santa Clara, Calif., IBM presented the first data about CMP processes. In 1993 at the VMIC Conference, IBM showed that a copper damascene (laying metal lines in an insulator trench) process was feasible for the MLM requirements with CMP processing steps. In 1995 the first tungsten polishing slurry was commercialized.
The National Technology Roadmap for the Semiconductor Industries (1994) indicates that the current computer chips with 0.35 micron feature sizes will be reduced to 0.18 micron feature size in 2001. The DRAM chip will have a memory of 1 gigabit, and a typical CPU will have 13 million transistors/cm2 (currently they only contain 4 million). The number of metal layers (the xe2x80x9cwiresxe2x80x9d) will increase from the current 2-3 to 5-6 and the operating frequency, which is currently 200 MHz, will increase to 500 MHz. This will increase the need for a three dimensional construction on the wafer chip to reduce delays of the electrical signals. Currently there are about 840 meters of xe2x80x9cwiresxe2x80x9d/chip, but by 2001 (without any significant design changes) a typical chip would have 10,000 meters. This length of wire would severely compromise the chip""s speed performance.
The global planarization required for today""s wafer CDs (critical dimensions) improves the depth of focus, resulting in better thin metal film deposition and step coverage and subsequently increases wafer yields and lowers the cost/device. It is currently estimated (1996) that it costs $xcx9c114/layer/wafer with current limited planarization processes. As the geometries become smaller than 0.35 micron, the planarity requirements for better lithography become critical. CMP is becoming important, if not essential, for multiple metal levels and damascene processes.
The CMP process would appear to be the simple rotation of a wafer on a rotary platen in the presence of a polishing medium and a polishing pad that grinds (chips away) the surface material. The CMP process is actually considered to be a two part mechanism: step one consists of chemically modifying the surface of the material and then in the final step the altered material is removed by mechanical grinding. The challenge of the process is to control the chemical attack of the substrate and the rate of the grinding and yet maintain a high selectivity (preference) for removing the offending wafer features without significant damage to the desired features. The CMP process is very much like a controlled corrosion process.
An added complexity is that the wafer is actually a complex sandwich of materials with widely differing mechanical, electrical and chemical characteristics, all built on an extremely thin substrate that is flexible.
The CMP processes are very sensitive to structural pattern density which will affect metal structure xe2x80x9cdishingxe2x80x9d and oxide erosion. Large area features are planarized slower than small area features.
At the recent SEMICON/Southwest 95 Technical program on CMP, it was stated that xe2x80x9cMetal CMP has an opportunity to become the principal process for conductor definition in deep submicron integrated circuits.xe2x80x9d Whether or not it does so depends on the relative success of CMP technologists in achieving the successful integrated process flow at competitive cost.
Slurries: CMP has been successfully applied to the planarization of interdielectric levels (IDL) of silicon oxides, BPSG, and silicon nitride and also metal films. The metal films currently being studied include tungsten (W), aluminum (Al) and copper (Cu).
The polishing slurries are a critical part of the CMP process. The polishing slurries consist of an abrasive suspension (silica, alumina, etc.) usually in a water solution. The type and size of the abrasive, the solution pH and presence of (or lack of) oxidizing chemistry are very important to the success of the CMP process.
Metal CMP slurries must have a high selectivity for removing the unwanted metal compared to the dielectric features on the wafers. The metal removal rate should be between 1700 to 3500 xc3x85/min) without excessive xe2x80x9cdishingxe2x80x9d of the metal plugs or erosion of the oxide substrate.
The oxide CMP has similar requirements and polishing rates close to 1700 xc3x85/minute.
Metal Polishing: This type of polishing relies on the oxidation of the metal surface and the subsequent abrasion of the oxide surface with an emulsion slurry. In this mechanism, the chemistry""s pH is important. The general equations are (M=metal atom): 
Under ideal conditions the rate of metal oxide (MOy) formation (Vf) will equal the rate of oxide polishing (Vp), (Vf=Vp). If the pH is too low (acidic) then the chemistry can rapidly penetrate the oxide and attack the metal (Vf less than Vp), thus exposing the metal without any further oxide formation. This means that all metal surfaces, at high points and in valleys, are removed at the same rate. Planarization of the surface is not achieved. This could cause metal plug connectors to be recessed below (xe2x80x9cdishingxe2x80x9d) the planarization surface which will lead eventually to poor step coverage and possible poor contact resistance.
When the pH is too high (caustic), then the oxide layer may become impenetrable to the chemistry and the metal becomes passive, (Vf greater than Vp) and the metal polishing rate becomes slow. Metal polishing selectivity to oxide generally ranges from 20 to 100:1, depending on the metal type. Tungsten metal should have selectivities  greater than 50:1 for the metal to oxide, and copper could have  greater than 140:1 metal to oxide selectivity. Etch rates can be up to 7000 xc3x85/min. The chemical diffusion rate and the type of metal oxide surface are important to the successful planarization process. A detailed mechanism has been proposed by Kaufman.
In practice, the low pH and highly corrosive oxidants (ferric nitrate) being used with an example metal CMP process has created corrosion problems with the polishing equipment. Currently the oxidant used in the metal polishing step has ranged from nitric acid to hydrogen peroxide, cesium and ferric nitrate solutions and even ferric cyanide solutions. Because of chemical stability problems, many slurries are made up at the point of use which means that there is little or no shelf life.
Metal planarization needs an oxidizing reagent that is stable and is not going to contribute to mobile ion contamination, will not xe2x80x9cstainxe2x80x9d the equipment, will not affect the slurry composition and slurry particle distribution and is generally environmentally friendly. The current hydrogen peroxide systems are not stable when premixed with the slurry and therefore have to be delivered to the polishing equipment with separate pumping systems and mixed at the point of use. The ferric nitrate system requires a low pH and is known to xe2x80x9cstainxe2x80x9d the polishing equipment. The potassium iodate system also requires special handling.
An emerging area of CMP will deal with the copper damascene process. The copper metal interconnects (wires) will be required because of its better conductivity compared to Al. One major disadvantage with copper is its easy diffusion through silica under normal operating conditions. The copper damascene process will need barrier layers to prevent this copper diffusion.
In the damascene process, xe2x80x9clinesxe2x80x9d or trenches are etched into the interdielectric layers, and then the walls of these trenches are coated with barrier materials. These materials can be composed of Ta, TaN, Ti or TiN among other materials. Copper metal is then deposited, by electroless or electrode plating, or PVD or CVD methods. The excess copper above the trench is then removed by chemical mechanical polishing. The difficult part of the CMP process is not to remove excess copper (xe2x80x9cdishingxe2x80x9d) which will remove the copper metal below the interdielectric layer.
CMP of the copper metal can be done over a wide pH range (2 to 12). Pourbaix diagrams for copper indicate that copper can only be passivated (oxide layer) in neutral or basic solutions. In acid solutions an inhibitor, i.e., benzotriazole (BTA) is usually needed to control the isotropic etching effects from the chemistries used in the CMP process. Much of the CMP work has been done with hydrogen peroxide at various pH ranges.
Some CMP work has been done with ammonium hydroxide, because of its ability to form copper complexes though there are problems with poor selectivity between copper and titanium and silicon oxide.
Interlayer Dielectric (Oxide) Polishing: Recently a group of engineers using ILD (oxide) CMP was asked to prioritize CMP processing requirements. The major concern was surface damage (scratching, etc.) followed by wafer (polishing) nonuniformity (within wafer and wafer to wafer), then polishing rate and finally planarity. The mechanisms are still being developed, but the polishing process appears to involve two concurrent processes; a mechanical process involving plastic deformation of the surface and, chemical attack by hydroxide (xe2x80x94OH) to form silanol bonds. 
In a slurry (colloidal suspension) the pH is important and for the silicon oxide system it needs to be in the 10 to 11.5 range. Currently CMP users are using silicon oxide-based slurries which were xe2x80x9cbufferedxe2x80x9d with sodium hydroxide but now are being formulated with potassium or ammonium hydroxide solutions. Etch rates can be in the range of 1700 xc3x85/min.
If the pH is too high the polynuclear species may start to precipitate in an unpredictable manner. There is also the possibility of a condensation process to form Si bonds.
There are other important features of the silicon surface that will influence the etch rates and final surface conditions; (metal contamination and possibly micro scratches). As mentioned above the typical silicon surface is terminated (covered) with xe2x80x94OH groups under neutral or basic conditions. The silicon surface is hydrophilic (the surface is xe2x80x9cwettablexe2x80x9d). These groups activate the surface to a number of possible chemical or physioabsorbtion phenomena. The Sixe2x80x94OH groups impart a weak acid effect which allows for the formation of salts and to exchange the proton (H+) for various metals (similar to the ion exchange resins). These Sixe2x80x94Oxe2x88x92 and Sixe2x80x94OH can also act as ligands for complexing Al, Fe, Cu, Sn and Ca. Of course the surface is very dipolar and so electrostatic charges can accumulate or be dissipated depending on the bulk solution""s pH, ion concentration and charge. This accumulated surface charge can be measured as the Zeta potential.
If the silica (Si) surface underneath the oxide layer is exposed because of an over aggressive polishing process, this could cause electrochemical problems because silica has a modest redox potential which will allow Cu, Au, Pt, Pb, Hg and Ag to xe2x80x9cplate onxe2x80x9d the silica surface. Exposure to light will also affect the redox reaction for Cu. The light will xe2x80x9cgeneratexe2x80x9d electrons in the semiconductor Si material which then reduces the copper ion to Cuxc2x0.
Post-Clean Processes: Both the ILD and metal polishing processes must eventually pass through a final cleaning step to remove traces of slurry and the chemistry. Though the process appears to be simple, i.e. a brush scrub and a rinse cycle, considerable effort is being expended to determine if the process should involve either single side, double sided scrubbing, single wafer or batch processing, spray tools or even immersion tanks. Recently an engineering group working with post-clean CMP ranked wafer cleanliness (from slurry and pad particles and metallic contamination) as the most important issue in the post-clean step. Process reliability and defect metrology were the other two important areas of concern.
Residual particle levels must be xcx9c1 particle/20 cm2, and 90% of these particles with less than  greater than 0.2 micron size. Line widths of 0.35 micron will require the removal of particles down to 0.035 or less. Incomplete particle removal will decrease wafer yield. Low defect (scratches) levels and acceptable planarity will also be very important.
Most fabs have developed their own in-house technology for the post-clean CMP steps. Most of the xe2x80x9cchemistriesxe2x80x9d involve DI water with either added ammonium hydroxide or HF while some fabs are using the standard RCA SC-1 (NH4OH:H2O2:H2O) and SC-2 (HCl:H2O2:H2O) cleaning steps traditionally used in the front end process.
There are five mechanisms for removing impurities (particles and/or ions) from wafer surfaces:
Physical desorption by solvents: Replacing a small number of strongly absorbed material with a large volume of weakly adsorbed solvent (changing the interaction of the surface charges).
Change the surface charge with either acids or bases: The Sixe2x80x94OH or Mxe2x80x94OH group can be protonated (made positive) in acid or made negative with bases by removing the proton.
Ion competition: Removing adsorbed metal ions by adding acid (i.e. ion exchange).
Oxidation or decomposition of impurities: Oxidation of metals, organic materials or the surface of slurry particles will change the chemical bonds between the impurities and substrate surface. The chemical reaction can either be through redox chemistry or free radicals.
Etching the surface: The impurity and a certain thickness of the substrate surface is dissolved.
In accordance with a first aspect of the invention, a composition for chemical mechanical polishing includes a slurry. A sufficient amount of a selectively oxidizing and reducing compound is provided in the composition to produce a differential removal of a metal and a dielectric material. A pH adjusting compound adjusts the pH of the composition to provide a pH that makes the selectively oxidizing and reducing compound provide the differential removal of the metal and the dielectric material.
In accordance with a second aspect of the invention, a composition for chemical mechanical polishing is improved by including an effective amount for chemical mechanical polishing of a hydroxylamine compound.
In accordance with a third aspect of the invention, a composition for chemical mechanical polishing is improved by including ammonium persulfate.
In accordance with a fourth aspect of the invention, a composition for chemical mechanical polishing is improved by including a compound which is an indirect source of hydrogen peroxide.
In accordance with a fifth aspect of the invention, a composition for chemical mechanical polishing is improved by including a peracetic acid.
In accordance with a sixth aspect of the invention, a composition for chemical mechanical polishing is improved by including periodic acid.
In accordance with a seventh aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material. A selectively oxidizing and reducing compound is applied to produce a differential removal of the metal and the dielectric material. The pH of the slurry and the selectively oxidizing and reducing compound is adjusted to provide the differential removal of the metal and the dielectric material.
In accordance with an eighth aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of a hydroxylamine compound.
In accordance with an ninth aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of ammonium persulfate.
In accordance with a tenth aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of a compound which is an indirect source of hydrogen peroxide.
In accordance with an eleventh aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of a peracetic acid.
In accordance with an twelfth aspect of the invention, a method for chemical mechanical polishing comprises applying a slurry to a metal and dielectric material surface to produce mechanical removal of the metal and the dielectric material, and an effective amount for chemical mechanical polishing of periodic acid.